[Moli2211]

Purpose

Fixes a 400 on /v1/responses when a tool result is sent as a list of content parts instead of a string.

ResponsesRequest.input validates its items against the OpenAI SDK types, and there FunctionCallOutput.output is Union[str, ResponseFunctionCallOutputItemListParam], so the list form is a valid request. response_input_to_harmony passed that value straight into Message.from_author_and_content, which expects a string, so the request failed with:

1 validation error for Message
content.0
  Input should be a valid dictionary or instance of Content
  [type=model_type, input_value=[{'text': '...', 'type': 'input_text'}], input_type=list]
  https://errors.pydantic.dev/2.13/v/model_type

The list form is what a client produces when a tool result arrives as content blocks, which is the normal case with MCP tools, and the same request is accepted by the OpenAI Responses API, so a client that works there fails here.

The Chat Completions path already handles this: #25593 added list flattening for the tool role in parser/harmony_utils.py, fixing #25001, which reported the identical error. The Responses path was left as it was. flatten_input_text_content is already imported in responses/harmony.py and already used there for system and developer messages, so one call makes both surfaces behave the same.

After the change a string output passes through unchanged, a content-part list is joined into a single string, and an empty list becomes "", matching Chat Completions. Non-text parts (input_image, input_file) are dropped, which is also what Chat Completions does today; widening that is out of scope here.

Reproduced on v0.27.1, and the code path is identical on v0.28.0 and main.

Test Plan

Three unit tests in tests/entrypoints/openai/responses/test_harmony_utils.py::TestResponseInputToHarmony covering string output, content-part list output and empty list output, modelled on the existing test_tool_message_with_array_content for response_previous_input_to_harmony.

pytest tests/entrypoints/openai/responses/test_harmony_utils.py

Test Result

Whole module, on a CPU-only checkout with vllm imported as a namespace package, since the module under test is pure Python while the repository conftest.py needs a built vLLM.

Before the change:

2 failed, 86 passed
FAILED TestResponseInputToHarmony::test_function_call_output_with_list_output
FAILED TestResponseInputToHarmony::test_function_call_output_with_empty_list_output
E   Input should be a valid dictionary or instance of Content [type=model_type,
    input_value=[{'type': 'input_text', ...}], input_type=list]

After the change:

88 passed

ruff check and ruff format --check are clean on both files with the pinned v0.14.0.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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🚀

[Manny7717]

APPROVE — verified locally on head fe945f9 (base cdefd9d).

Bug confirmed on base: ResponsesRequest.input accepts FunctionCallOutput.output as Union[str, ResponseFunctionToolCallOutputItemListParam] (OpenAI SDK types), but response_input_to_harmony passed it raw into Message.from_author_and_content (string-only). Executed probe on base: output=[{"type":"input_text","text":"Paris "},...] → ValidationError: content.0 Input should be a valid dictionary or instance of Content — exactly the reported 400.

Fix verified on head: flatten_input_text_content(output) or "" — same helper + same or "" idiom the Chat Completions path uses (parser/harmony_utils.py, added #25593 for #25001). Probe matrix on head:

• string "sunny" → "sunny" (passthrough, base-valid form unchanged)
• list [{input_text "Paris "}, {input_text "sunny"}] → "Paris sunny" (the fix)
• [] → "" (matches Chat Completions behavior)
• [{input_image}] (no text parts) → "" (flatten returns None → or "")

Consistency check (fix-the-class): all sibling message types in this function already route through flatten_input_text_content (system/developer at :113/:163, chat_msg at :126); function_call_output at :188 was the only branch missing it. No other response_msg["output"] consumption sites. Grep-verified.

Tests: new TestResponseInputToHarmony class — 3/3 pass on head (string, list, empty-list). The teardown RuntimeErrors in the file are the known CUDA-less torch.accelerator.memory env noise (fixture teardown, not assertions; identical class of noise on base).

Minimal, correct, consistent with the established pattern. No competing PRs (searched).